This is a Phase III randomized multicenter trial to evaluate the safety and tolerance of ZDV monotherapy vs d4T monotherapy administered for 52 weeks to antiretroviral-naive symptomatic HIV-infected infants. Clinical response variables include disease progression based on growth failure, changes in neurodevelopment status, or development of two or more new or recurrent AIDS-defining opportunistic infections. Children will be evaluated for toxicity monitoring at baseline and every 4 weeks thereafter. Based on the recommendations of the NIAID Data and Safety Monitoring Board and the ACTG Pediatric Executive Committee, all patients receiving ZDV monotherapy on ACTG 152 were unblinded and discontinued from that arm. Subsequently, the ACTG 240 team and the ACTG Pediatric Executive Committee met and agreed that all patients enrolled in ACTG 240 would be unblinded to randomized study treatment. ACTG 240 patients were offered best available therapy based on discussion between the physician and the family.